One of such ultrasonic testing devices as described above is known where a sensor head is provided comprising two or more of the transmitters and the receivers disposed at both ends of the center point and particularly held at a uniform angle (the incident angle) to the test piece for scanning the surface of the test piece with no direct contact (See Patent Document 1). It is essential for maintaining the incident angle to a uniform degree to match different modes of the relationship between the incident angle of the ultrasonic wave and the product of ultrasonic frequency and test piece thickness with their respective characteristic curves.
Patent Document 1: Japanese Patent Laid-open Publication No. 2005-055197.
However, if the surface of the test piece is undulated or waved, the incident angle between the transmitter or receiver and the surface of the test piece may hardly be maintained at a uniform degree during the scanning action with no direct contact of the sensor head, thus interrupting the inspecting action with the plate wave.
Alternatively, another testing method is known for inspecting a test piece with the transmitter and receiver placed directly on the test piece. However, the another method also fails to propagate the plate wave up to the receiver after the point on the propagation path of the plate wave where the transmitter or receiver is placed directly on the test piece, hence permitting no use of two or more of the receivers.
More specifically, the conventional methods allows the transmitter or receiver to be disposed across the propagation path of the plate wave which extends through the target region of a test piece to be inspected when the inspection depend fundamentally on the leak wave or the reflection of the ultrasonic wave. As a result, the degree of freedom of the testing action will be limited due to a limited target area of the test piece. Also, with no use of two or more transmitters or receivers, the testing action may hardly be improved in the efficiency.